It is well known that the electrical sparks fed to the spark plugs of an internal combustion engine are usually produced by means of an ignition coil having its high voltage secondary winding connected to the engine's spark plug through a distributor, the coil having its primary winding connected to a low voltage source, typically a 12 volt battery or an alternator system driven by the engine. An engine driven switching device, typically a mechanical contact breaker produces interruptions in the current flowing the primary winding of the coil and consequentially high voltage pulses are produced in the secondary winding, which are applied to the spark plugs.
Considerable research has been directed at improving fuel economy and reducing pollutant emission of internal combustion engines. Efforts have been made to devise an engine which will run satisfactorily with a leaner fuel to air ratio fed to the cylinders of the engine. Such a leaner fuel to air ratio reduces fuel consumption but has the disadvantage that the fuel/air mixture becomes more difficult to ignite with a conventional spark ignition system. Also, the increased ratio of air to fuel increases the likelihood of nitrogenous fuel constituents being converted by the combustion into oxides of nitrogen, hereinafter referred to as NO.sub.x. Such NO.sub.x products are difficult to extract from the exhaust of the engine, as is necessary if the engine is to comply with pollutant emission regulations.
In order to reduce NO.sub.x exhaust emission, recent research has been directed to recirculating engine exhaust gases into the engine's cylinders so as to reduce the ratio of fuel to air in the mixture to be consumed in the engines, whilst maintaining a lean fuel content in the mixture. Such exhaust gas recirculation, hereinafter referred to as EGR, reduces NO.sub.x emission but cools the temperature of combustion and makes the combustible mixture in the engine's cylinders even more difficult to ignite.
Various proposals have been made to overcome the difficulties of igniting a lean fuel/air mixture. One solution involves the redesign of the engine such as to produce a so-called stratified charge in the cylinders. In a stratified charge engine, a fuel/air mixture has a non uniform spatial fuel distribution within the cylinder such that a higher concentration of fuel occurs adjacent the spark plug than in the most part of the cylinder. When spark ignition occurs, combustion will occur more readily in the relatively high fuel concentration adjacent the spark plug and the ensuing heat of combustion will cause the combustion to spread to the leaner mixture in the other parts of the cylinder. One example of a stratified charge engine is described in "A New Concept of Stratified Charge Combustion--The Ford Combustion Process (FCP)"SAE Paper No. 680041 January 1968. This engine was developed into the PROCO engine described in "The Ford PROCO Engine Update" SAE Paper No. 780699 August 1978. It will be seen that in contrast to the FCP engine, the PROCO engine has an EGR system to reduce NO.sub.x exhaust emissions. Moreover it is to be noted that in order to achieve satisfactory combustion, two spark plugs per cylinder are required with the EGR assisted PROCO engine, thus illustrating the further difficulties that occur in initiating ignition of a lean burn mixture when EGR is used. It will be appreciated that when two spark plugs per cylinder are used, a complex distributor is required and the overall cost of the ignition system is increased substantially. Another problem resulting from the use of two spark plugs per cylinder is that the PROCO engine concept can only be used for large engine capacities of typically 5 liters or more. For smaller engine capacities, there is not enough room in the cylinder head to receive the spark plugs and the necessary valves and injectors used for this type of engine.
Another proposal for igniting a fuel/air mixture is shown in U.S. Pat. No. 4,033,316 to Birchenough, that discloses an arrangement in which a high voltage direct current source is connected in series with the secondary winding of an ignition coil, in such a manner as to maintain the spark initiated by the conventional operation of the coil. Thus, the spark ignition in the cylinder is initiated by a typically 20 KV pulse produced in a conventional manner by interrupting the current flow in the coil's primary winding, and the spark is thereafter maintained by a high voltage of typically 2 to 4 KV from the high voltage d.c. source that is connected in series with the coil's secondary winding, in a manner broadly analogous to the way in which a welding arc is initiated by a high voltage pulse and is sustained by a lower voltage direct current. It is well known that once an arc has been struck it can be maintained by a voltage less than that required to strike the arc.
Birchenough states that the voltage required to sustain the spark is generally constant, and that the voltage current characteristic of the direct current generator should be such as to deliver a constant current to the spark.
The circuit shown in FIG. 2 of the Birchenough U.S. patent achieves this condition by arranging the d.c. generator to have an output voltage current characteristic defined by a curve for which a decreased output voltage results in an increased current, the maximum current at low voltage being limited by the output impedance of the d.c. generator.
I have found that in practice, whilst the voltage required to sustain the arc does have a generally constant mean value, it is subject to transistory fluctuations, these fluctuations occurring during combustion conditions of high EGR, high compression, high gas swirl rates within the cylinder, and extremely lean burn fuel mixtures. During such a transistory fluctuations both a relatively high voltage and current may be required to sustain the spark. However, the d.c. generator of Birchenough delivers a relatively low voltage at high current levels and accordingly will not sustain the spark during such transitory conditions unless the d.c. generator is made more powerful and accordingly inefficient for the most part of its operation.
Another problem occurs with the d.c. generator of Birchenough in the event that a short circuit occurs across the spark plug or across the output of the generator. Such a short circuit condition could occur when an engineer is checking operation of the ignition circuit. It is common practice to touch the spark plug's lead against the engine to see if a spark jumps from the end of the lead. During such testing a short circuit is likely to occur across the output of the d.c. generator. Now, the d.c. generator of Birchenough comprises a free running oscillator which drives a step up transformer, the output of which is applied to a diode and capacitor network which acts as a rectifier and voltage multiplier, to develop a high voltage d.c. output across an output capacitor. In the event of a short circuit across the output, the d.c. generator operates to pump a high current into the short circuit. As a result the oscillator will overheat and is likely to fail. The circuit of Birchenough is accordingly dangerous to maintenance engineers. If an engineer accidentally touches the high voltage spark plug lead, to produce a short circuit, the d.c. generator will pump a heavy current into the short, with consequential hazardous results for the engineer.
Another problem with the Birchenough circuit is that the operation of the high voltage d.c. source produces substantial erosion rates of the spark plugs' electrodes and the electrodes of the distributor and its associated rotor arm. This problem is particularly serious if the energy supplied by the d.c. generator is selected to be high enough to maintain the spark during combustion with high EGR rates and high combustion gas swirl for lean burn fuel mixtures.
A further problem with the Birchenough circuit is that the output capacitor of the d.c. generator will remain charged for a substantial time after the circuit has been switched off. Thus, if an engineer touches the output of the ignition circuit even after the circuit has been switched off, he is liable to receive an electric shock.